Suppose a druid has used wild shape and is still in the shape-shifted form, but wants to do so again. Maybe to switch to a different form or maybe just to regain the temporary hit points. Can they do so normally with a single bonus action? Or do they need to first use one bonus action to leave the current form and then another bonus action (presumably on their next turn) to activate the new one?

Yes, Wild Shape say you stay in that form for a number of hours equal to half your Druid level or until you use Wild Shape again.

I believe that was possible with the 2014 rules as well.

Quote from Frachese >>

Suppose a druid has used wild shape and is still in the shape-shifted form, but wants to do so again. Maybe to switch to a different form or maybe just to regain the temporary hit points. Can they do so normally with a single bonus action? Or do they need to first use one bonus action to leave the current form and then another bonus action (presumably on their next turn) to activate the new one?

 I recalled your idea was indeed highlighted in the pre-release article The 2024 Circle of the Moon Druid and Changes to Wild Shape:

Staying in Wild Shape

One of my personal biggest frustrations with playing a 2014 Druid at higher levels is that during combat, sometimes I’d spend my turn to shift into a Beast form, then before my next turn came around in the Initiative order, I’d take enough damage to push me back out of it. As a result, I’d ostensibly lost a turn and expended a pretty finite resource.

Although you no longer get a separate pool of Hit Points by shape-shifting, getting knocked out of Beast form is harder as you retain your Druid's Hit Points. You also get Temporary Hit Points to act as a buffer.

The 2024 Wild Shape rules really come from a place of understanding that shape-shifting into a Beast is a major appeal of the Druid class. By making it easier to stay in Beast form and allowing you to expend spell slots to replenish a usage of Wild Shape, the 2024 Player’s Handbook is really geared toward making you eager and excited to use this ability.